This invention relates generally to the testing of kinescopes during production and particularly to a method of and apparatus for testing for electrical breakdown of the frit seal of such tubes.
Kinescopes for television include a funnel, a neck and a faceplate. Typically, the funnel and neck are integrally fabricated with the neck portion attached to the narrow end of the funnel. The faceplate is affixed to the wide end of the funnel by a fritting process which hermetically couples the two elements. The inside of the funnel and a portion of the neck are covered with a conductive coating which is biased with a high positive voltage during the operation of the tube to accelerate the electron beams toward the faceplate. A problem sometimes occurs with the frit seal when the electrical coating material diffuses into the fritting material or when contaminates on the funnel or faceplate render the frit seal electrically conductive making the tube unacceptable. Every envelope must, therefore, be checked for this condition prior to further processing.
Every kinescope is provided with an external terminal which is electrically connected to the internal conductive coating so that the coating can be positively biased. This terminal is called the P2, or anode button. In the prior art when testing for electrical frit breakdown, a potential in excess of the normal operating potential is applied to the anode button terminal. A conductive band is placed around the frit seal so that an electrical breakdown at any point along the seal is electrically coupled to the band. The band is then either grounded or connected to the negative side of the high voltage power supply. An electrical frit breakdown is then indicated by a current flow when the high voltage is applied. In some tubes, particularly those with smaller diagonals, such as 17 and 19 inches (43 and 48 cm), the spacing between the anode terminal and the frit seal is close and arcing occurs between the band and the connector which couples the high voltage to the anode terminal. Such arcing results in a current flow and thus is undistinguishable from a frit breakdown to the testing equipment so that good tubes appear to be failures.
The instant invention addresses this problem by the provision of a probe which is inserted into the neck of the kinescope so that the test voltage is applied directly to the inner conductive coating electrode through a movable contact. The possibility of arcing between the connector and the band around the frit seal is thus eliminated.